1. Field of the Invention
This invention relates to a supercharging device of an engine which is provided with a mechanical supercharger.
2. Description of a prior art
As a supercharging device of an engine, such a device as disclosed by Japanese Utility Model Registration Application Laying Open Gazette No. 61-78250, for example, has been known. According to this device, a mechanical supercharger which is driven by an engine is provided in an air intake passage downstream from a throttle valve, and a supercharging control valve is provided in the air intake passage between the mechanical supercharger and the throttle valve, whereby when a supercharging pressure reaches a set point, the supercharging pressure is maintained at that set point by adjusting the opening of the supercharging control valve so as to rationalize supercharging characteristics.
Another supercharging device as disclosed by Japanese Utility Model Registration Application Laying Open Gazette No. 61-17138 is such that a mechanical supercharger which is driven by an engine is provided in an air intake passage, a bypass which bypasses the mechanical supercharger is provided at the air intake passage, a throttle valve is provided in the air intake passage upstream of a joint of the bypass on an intake side of the supercharger, and a diaphragm type control valve which opens in response to an air intake negative pressure is provided in the bypass. According to this supercharging device, when the required output is small and negative pressure at the downstream of the throttle valve is large, the control valve opens, whereby supercharged air downstream from the supercharger is supplied upstream of the supercharger via the bypass so as to relieve negative pressure of intake air and prevent a temperature rise in the air intake passage. On the other hand, when the required output is large and a supercharging pressure downstream of the supercharger is also large, the control valve which has a function of a check valve opens by application of the supercharging pressure so as to relieve the supercharged air downstream from the supercharger to upstream of the supercharger via a bypass and prevent an excess of supercharging pressure.
In the above supercharging device provided with a mechanical supercharger, a bypass and a control valve in an air intake passage, it is suggested to provide a clutch between the mechanical supercharger and an output shaft of the engine so as to put the mechanical supercharger in a non-connected state, when the required output is small, by turning the clutch "OFF" and thereby supply intake air to the engine through the bypass, and when the required output is large, to put the mechanical supercharger in a connected state by turning the clutch "ON" and thereby supercharge the engine. With the above arrangement, when the required output is small, power to be absorbed by the mechanical supercharger becomes zero, with the result of a reduction in fuel usage, and when required output is large, it is possible to secure engine output.
However, in the above supercharging device with a clutch, when the clutch in the "OFF" state turns "ON" with the increase of required output and the mechanical supercharger is switched from a "non-connected state" to a "connected state", the mechanical supercharger is driven suddenly and the flow rate of the discharge air of supercharger increases. In this case, due to an overcharged pressure downstream from the supercharger, a control valve opens and supercharged air is relieved to upstream of the supercharger through the bypass. Therefore, a sound generated by the variation of pressure in the supercharger and other factors is transmitted to the upstream side of the supercharger as it is carried by the flow of the relieved air, with the result that a noise passes through the air intake passage and is emitted to the exterior.